Analog eddy current proximity systems which analyze and monitor rotating and reciprocating machinery are known in the art. These analog systems typically include a proximity probe located proximate a target object (e.g., a rotating shaft of a machine or an outer race of a rolling element bearing) being monitored, an extension cable and analog conditioning circuitry. The target, proximity probe (a noncontacting device which measures displacement motion and position of an observed conductive target material relative to the probe), extension cable and conditioning circuitry components are all designed to interact in such a way that a voltage output from the circuitry is directly proportional to a distance between the probe and the target. This distance is commonly referred to as “gap”.
The interaction that takes place between these components is in accord with the following rules: First, the electrical impedance measured at the conditioning circuitry is the electrical combination of the target, the probe including an integral sensing coil and cable, the extension cable and the conditioning circuitry. This impedance is usually called the “Tank Impedance” or parallel impedance (Zp). Second, this tank impedance is linearized and converted into a voltage directly proportional to gap. Third, the conditioning circuitry measures impedance at a specific frequency that is a function of its own circuitry. Generally, the circuitry runs at the frequency where the reactive component of the tank impedance approaches zero. In other words, the circuitry is a resonant system, so the frequency of operation will be where the phase shift of the impedance is approximately zero degrees. In reality, the phase shift is not exactly zero due to, inter alia, manufacturing and component variations and tolerances of each analog system.
In order to compensate for these variations and tolerances, each analog system is required to be calibrated to have a parallel impedance which is as close as possible to a predefined ideal parallel impedance while remaining substantially unsusceptible to the multitude of variations and tolerances found in the target, probe, extension cable, and conditioning circuitry. Simultaneously, each analog system is calibrated to have a maximum sensitivity to changes in gap. Moreover, each system is generally required to be calibrated to monitor one specific target material.
These analog systems are also generally burdened by temperature variations in the target, the probe including the integral sensing coil and cable, the extension cable and the conditioning circuitry due to the severe temperature variations in rotating and reciprocating machinery environments. Thus, each system is required to be designed around a multitude of component tolerances to compensate for the severe temperature variations engendered in these environments. Furthermore, these analog systems must also be designed around the sensitivity to changes in the conductivity and permeability of the target, the sensing coil, and the cable, which can greatly effect the precision of these systems.
Moreover, interchangeability problems arise from variations in the target, probe, extension cable, and conditioning circuitry which cause the tank impedance (Zp) versus gap to vary slightly from nominal resulting in a proclivity towards, inter alia, variations in incremental scale factor (ISF), variations in average scale factor (ASF) and deviations from a straight line (DSL). The incremental scale factor (ISF), variations in average scale factor (ASF) and deviations from a straight line (DSL) are common ways to specify transducer performance as is well known in the art.
It is critical that the displacement motion or position between the target and the sensing coil of the proximity probe remains within the linear range of the proximity probe for providing accurate and reliable measurements over a wide range of circuit and environmental conditions in order to operate rotating and reciprocating machinery safely and efficiently. Heretofore, the ability to provide accurate and reliable measurements over a wide range of circuit and environmental conditions has been dependent on, inter alia, designing and manufacturing each production unit within close tolerances and going through laborious calibration methods to compensate for the circuit and environmental conditions.
For the foregoing reasons, there is a need for an eddy current transducer system that, inter alia, substantially eliminates the manufacturing and component variations and tolerances of the prior art analog systems, a system that provides correct gap reading for different target materials and a system which is easy to calibrate.
Additionally, there is a need to solve the general problem of compensating for temperature errors, temperature profiles of different target materials and changes in component conductivity and permeability in order to preclude anomalous behavior in eddy current transducer systems.
Furthermore, there is a need for an eddy current transducer system that has better linearity and interchangeability. Moreover, there is a need for an eddy current transducer system that does not require component changes when re-calibrated to a new or different target material.